Removing bismuth from lead



Patented Sept. 3, 1940 UNITED STATES PATENT OFFICE 2,213,197 REMOVING BISMUTH FROM LEAD No Drawing.

20 Claims.

This invention relates to the refining of metals,

'and more particularly, to the removal of bismuth from desilverized lead.

It is now well-known that the bismuth content of lead may be reduced to low limits through the use of certain alkali, alkaline earth metal reagents.

It is the primary object of the present invention to provide a process for economically effecting the further removal of bismuth from lead containing alkali, alkaline earth metal reagents or combinations thereof.

Other objects, features and advantages of the invention will be apparent from the following particular description.

As the process of the invention contemplates the treatment of lead containing, in addition to bismuth, debismuthizing reagent selected from the group consisting of alkali and alkaline earth metals including magnesium, the normal starting material for theapplication of the new process will be lead which has been debismuthized by reagents from the group just mentioned. This initial debismuthization may be well accomplished in accordance with the following procedures.

A cast-iron metallurgical kettle is filled with molten lead which has been recently desilverized in a manner well-known in the art. This is maintained at a temperature of approximately 700 F., and the oxide dross is removed.

'The particular reagent or combination of reagents from the alkali, alkaline earthgroups, for example, calcium, is applied to the bath as a lead-alkaline earth metal alloy, e. g. a lead-calcium or 'lead-calcium-magnesium alloy, and stirred into the bath, which is maintained at a temperature of about 660 F., until the bath has become uniform; r

After the desired mixture has been obtained, the dross containing the major portion of the bismuth may be removed by skimming, and treated in any desired manner for the recovery of the metal values therein.- The kettle then is slowly cooled and blocksand rims are formed from the portion of the metal which first solidifies. .These block and rims contain the remainder of the bismuth that separates.

The bath now contains, for example, from about 0.03% to about 0.05% of calcium and about 0.045% of bismuth, and the present invention provides an economical process for removal of substantially all of this remaining bismuth without introducing contaminating materials in the lead.

Application November 7, 1939, Serial No. 303,210

In the removal of bismuth from lead with any of the alkali and alkaline earth reagents, or combination of the same, there are solubility limits for any given temperature below which bismuth is not removed. However, in the practical applil cation of these-reagents to debismuthizing lead, it is impossible to reach these solubility limits, owing to the difficulty of physically separating the resulting dross from the metal.

In practicing the invention, the dross resulting 10 from the above-described treatment is so modified that it will separate readily from the metal and enable bismuth finals to be reduced to the actual solubility limits for the particular temperature. For example, at 640 F., after debisl5 'muthizing a lead with alkali, alkaline earth reagents, a two-phase system exists, that is, molten lead with entangled alkali and alkaline earth compounds with bismuth, and also with lead. That is to say, all the bismuth that is going to 20 be removed from the lead is present in the solid state as such compounds, and the modifying reagent which is added in the present process constitutes in efiect a means of changing the physical characteristics of these frozen compounds 25 from a wet metallic mixture to a dry mixture and thus enables a complete separation from the liquid lead to be accomplished readily.

Investigations made .in connection with this subject matter indicate that when, for example, calcium is used as the debismuthizing reagent, after the separated dross has been removed, the bath contains residual calcium, as has been pointed out above, together with a little residual bismuth (about 0.04%-0.05%) which does not 36 separate, and which is combined with a part of the residual calcium as calcium bismuthide, the remainder of the residual calcium being uncombined except with lead. It will be obvious that the total amount of this residual calcium is,de- 40 pendent upon the original amount incorporated in the bath and the original bismuth content of the bath. It seems from the results of studies that this is the most likely condition prevailingin the lead bath after the removal of the bis- 45 muth-containing crusts and rims, although it is to be understood that the actual true condition is not known definitely; but whatever it is, the same condition apparently is existent in the case of other alkaline earth compounds, including 60 magnesium compounds, and likewise in the case of sodium or potassium compounds.

However, this theory provides a ready explanation for the observed results. Thus, if chlorine is passed into the bath, calcium is removed, but 56 not bismuth, as the chlorine reacts with all cal cium with which it contacts, whether it be free calcium dissolved in the lead, or calcium-lead compound frozen out, or calcium bismuthide frozen out; and of course chlorine attacks the lead of the bath preferentially to the bismuth. Reagents such as carbon monoxide, carbon dioxide, oxygen, sulphur, fail to remove any ap-- preciable quantity of either calcium or bismuth at the low temperature of operation, even though most of the calcium may be present as free calcium and presumably in a favorable state for easy combination. Using such materials there is obtained a wet dross on the surface of the lead, but no appreciable calcium or bismuth re moval is effected; and all of the above results are obtained with whatever alkali, alkaline earth reagent was employed for debismuthizing purposes.

Now, it has been found that certain classes of organic compounds when added to a previously debismuthized lead bath do reduce the residual bismuth and calcium contents to a very marked extent. Such organic compounds are found to be, in general, proteins, illustrated by glue, casein, dried blood; carbohydrates, illustrated by starch, flour, sugar, cellulose, gum arabic; and aliphatic polycarboxylic acids or. acids yielding carbon on thermal decomposition, illustrated by tartaric acid.

It has been found that the addition of these reagents to a previously debismuthized bath is to remove both residual debismuthizing agents (e. g. Ca, Mg, K) and bismuth, the decomposition products of these reagents at the temperature of the bath (640 F.) apparently combining selectively, however, rather reluctantly, with the free reagent (calcium) but not that calcium which is in combination .as compounds with bismuth, thereby creating a condition which causes calcium bismuthide to become physically separable. There is obtained in practice a dry dross on the bath which may be skimmed ofi readily, the removal of the bismuth being accomplished to a value approaching that of ultimate true solubility of bismuthide bismuth in the bath for the particular alkaline reagent content.

The invention may be illustrated by the following specific examples.

PROTEIN REAGENTS 1. Glue Two hundred parts by weight of debismuthized lead assaying Bi 0.029%, Ca 0.048%, Mg 0.053%, were melted in a kettle, with five additions of glue of 0.22 part by weight each, making a total of 1.1 parts by weight of glue.

Each addition of glue was made into the vortex of a mechanical mixer introduced into the bath, the temperature of the bath being maintained continuously at 630635 F.

A dry black dross separated, which was skimmed at 650 F. and which weighed 46.6 parts by weight. The final metal (154.0 parts by weight) assayed Bi 0.016%, Ca 0.033%, Mg 0.029%.

2. Blood Two hundred parts by weight of debismuthized lead, assaying Bi 0.029%; Ca 0.048%; Mg 5 were treated at 630-635 F. with dried slaughterhouse blood, in accordance with the procedure under (1) above, a. total of 1.1 lbs. of blood being used.

The dross resulting from the treatment was skimmed 011 the bath at 650 F., and there were recovered 32.4 parts by Weight thereof.

The final metal, 168 parts by weight, assayed Bi 0.016%; Ca 0.030%; Mg 0.038%.

3. Casein Two hundred parts by weight of debismuthized lead, assaying as above, were treated with 1.1 parts by weight of casein in the above-described manner.

Thirty-eight parts by weight of dry black dross were skimmed from the bath, the final metal, 161 parts by weight, assaying Bi 0.019%; Ca 0.030%; Mg 0.032%.

CARBOHYDRATE REAGENTS 1. Gum arabic Two hundred parts by weight or debismuth- ,ized lead, assaying Bi 0.029%; Ca 0.048%; Mg

0.053%, were treated at 630635 F. with 1.1 parts by weight of starch, added in five portions of 0.22 part by weight. The resulting dross was skimmed at 650 F., there being formed 38.9 parts by weight of dry black dross. The final metal, 169 parts by weight, assayed Bi 0.017%; Ca. 0.030%; Mg 0.031%.

3. Sugar The above test was repeated, using 1.1 parts by weight of cane sugar, added in five portions of 0.22 part by weight each. There were removed 3037" parts by weight of dry black dross and the final bath, 168 parts by weight, assayed Bi 0.013%; Ca 0.038%; Mg 0.035%.

4. Corn starch.

Two hundred parts by weight of debismuthized lead, assaying Bi 0.055%; Ca 0.082%; Mg none, were melted in a kettle and 1.1 parts by weight of corn starch were added in five additions of 0.22 lb. each, at 630-'635 F., the additions being made to the vortex of a mechanical mixer operating in the bath.

The resulting dross was skimmed at 650 F., 36.9 parts by weight of dross being separated. The final bath, 163 parts by weight, assayed Bi 0.019%; Ca 0.006%.

5. Saw-dust Two hundred parts by weight of debismuthized lead, assaying Bi 0.029%; Ca 0.048%; Mg 0.053% were melted in a kettle-and 1.1 parts by weight of saw-dust, were added in five additions of 0.22 lb. each, at 630-635 F., the additions being made to the vortex of a mechanical mixer operating in the bath.

A dry black dross, 48.1 parts by weight, was skimmed from the bath, the final bath assaying Bi 0.012%; Ca 0.034%; Mg 0.021%.

CABBoN YIELDING ALIPHA'IIG POLYCABBOXYLIC Acm 1. Tartarz'c acid Two hundred parts by weight of lead, assaying as above, were treated in the same manner with 1.1 parts by weight of tartaric acid added in five additions of 0.22 part by weight each.

There were separated from the bath 29.2 parts by weight of dry black dross. The final metal bath assayed Bi 0.017%; Ca 0.034%; Mg 0.024%.

The e may be various explanations for these observ d results.

One of these explanations may be predicated upon the well-established observation that when bismuth-eontaining lead has its bismuth con tent reduced to say, 0.05% by suitable debismuthizing reagents, for instance, calcium, arepeated reheating and liquation of the debismuthized lead produces a small amount of a dross which contains, however, only very minor amounts of the residual calcium and of the residual bismuth. Consequently, it may be considered that in the reduction of the bismuth to to the bath combines. with the bismuth therein to form calcium bismuthide and that additional amounts of calcium, or for that matter some of the calcium added to take care of the bismuth, forms a compound with lead, calcium plumbide; and it is established also that calcium bismuthide when added to non-calcium-containing lead, dissolves therein, but when the lead already contains calcium, the solubility of the calcium bismuthide is decreased, the amount of such decrease being dependentupon the amount of calcium that is present in the bath. Furthermore,-

in the presence of magnesium in the bath, the solubility of the bismuthide is further very greatly decreased. It may be that, in the carrying out of the debismuthizing procedure, the calcium bismuthide content at the working equilibrium which is reached in present operating practice, is in the form of a colloidal suspension, or at least that amount which is present above the value of the ultimate equilibrium.

Now it will be observed that all of the operative reagents noted herein for reducing the hismuth content below that value obtained usually in practical operations by alkaline debismuthizing agents, are such that will deposit carbon upon thermal decomposition of the material; and it will be noted further that the dry dross produced is black in each case. These reagents also are thermally decomposable at low temperatures to deposit such carbon, and as the addition of these materials to the bath is made at low temperatures,- for example 630 F. to 640 F., the liberat d nascent carbon would be highly active, with high adsorption properties, to adsorb upon it the calcium bismuthide suspensoids in the bath, thereby producing the dry tain this black dross which was observed in each case, and reducing the residual bismuth content to the point of its ultimate equilibrium for the particular alkaline reagent content.

Another explanation may be chemical, rather than physical, in character.

As it has been mentioned previously, calcium and lead form a compound, calcium plumbide, CaPba. In a lead bath, this calcium plumbide forms a wet mushy dross, cleanly separable from the lead of the bath only with difiiculty, and containing substantial amounts of entrained metallic lead from the bath, the removal of the dross resulting in substantially lead losses from the bath. This result is observed whenever calcium is added to a bath of molten lead, even though that bath be bismuth-free.

In view of the spongy nature of calcium plumbide, and the presence thereof in a debismuthized lead bath, in accordance with this explanation, it will mechanically restrain the calcium bismuthide present in solid phase at the so-called working, equilibrium of the bath, from separating from the bath, but will maincalcium bismuthide disseminated throughout the bath.

Now, in accordance with this explanation, when the herein-described addition of the organic compounds are made, the active nascent carbon produced by the thermal decomposition of the materials react with the calcium combined with the lead, to break up the spongy calcium plumbide to release the lead, with formation of calcium carbide, thereby releasing the calcium. bismuthide and enabling the same to separate from the bath as a dry dross.

Evidence supporting this explanation lies in the reduction of the calcium (and magnesium) content of the bath along the reduction of the bismuth content; and the observed reactivity of the dross with water with an accompanying evolution of acetylene and hydrogen.

That the calcium bismuthide is present in the solid phase at the working equilibrium is evidenced by the fact that the addition of reagents at more elevated temperatures is inef-' fective for the removal of bismuth, and under no conditions does any calcium bismuthide separate from the bath until the latter is cooled to substantially the freezing point ,of the lead. Consequently, the present invention is practical at temperatures asv close to the freezing point of lead as is possible to operate. The temperature of the bath during treatment should not exceed substantially 640 F.

In accordance with the present invention, the final bismuth content of the lead is reduced easily to 0.025%, or less, predicated upon the amount of calcium'and magnesium introduced into the bath, results down to 0.0005% Bi being obtainable by the use of this process.

The efiect of the presence of the alkaline debismuthizing agents (calcium) is indicated in the following tables. These tables show the presence of calcium as the primary debismuthizing agent.

Five additions of starch 2.2 Ila/ton each, mia: 3 min. 630 F.

Original analysis After second addition After fifth addition Percent Percent Percent Percent Percent Percent B1 Ca i Ca Bi Ca 098 052 074 None. 120 042 098 None. 120 022 11 None.

REMovAL or BISMUTH FROM 0.055% BI LEAD CONTAINING VARIABLE QUANTITIES F CA Five additions of starch 2.2 Zb./ton each (100 gms. to 200 Zbs.lead) mizc 3 min. 630 F.

Original analysis After first addition Alter second addition After third addition After fourth addition After fifth addition Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent Bi Ca Bi Ca Bi Ce. Bi Ca Bi Ca Bi Ca 055 049 036 012 026 015 038 None .055 .030 025 .050 015 .002 015 None .055 020 .007 053 Tr. None 052 None However, whatever may be the true explanation of what takes place, the actual results are as have been set forth herein; and the present improvement comprises, generally speaking, a process of debismuthizing lead by means of the procedure of cooling the bath to about 640 F. or lower, while maintaining the bath molten, thereby causing the separation in solid phase of the bismuth to be removed, adding a modifying reagent to change the physical characteristics of bismuthcontaining compounds from a wet metallic state to a dry easily separable dross, and removing the dry dross from the bath.

What is claimed is:

1. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, efiecting a separation of bismuth in the lead bath, producing a dry bismuth-containing dross on the bath by incorporating therein a modifying agent for the bath which converts the bismuth from a difiicultly separable condition to a readily removable dry dross, and removing the dry dross from the bath.

2. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of lead, maintaining the bath at a temperature not greatly above the melting point of the lead, causing separation of the bismuth in a solid phase in the bath, and producing a dry bismuth-containing dross on the bath by incorporating therein a modifyi rg agent for the bath,

which converts the bismuth froni a wet difficultly separable condition to a dry dross which is readily removable from the bath.

3. The process of removing bismuth from bismuth-containing lead which comprises maintaining a molten bath of the lead at a temperature range of just above its melting point to not above approximately 640 F., causing separation of the bismuth in the bath, and producing a dry bismuth-containing dross on the bath by incorporating in the bath a modifying agent for the bismuth-containing dross which converts the latter from a wet condition to a dry condition.

4. The process 'of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, efiecting a separation of bismuth in the lead bath, and producing a dry bismuth-containing dross on the bath by incorporating in the bath a modifying agent for the 6. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, maintaining the bath molten but at a temperature not much above its melting point, effecting a separation of bismuth in solid phase in the bath, and producing a dry bismuth-containing dross by acting on the bath with nascent carbon produced therein.

'7. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, maintaining the bath at substantially as low a temperature as is consistent with fluidity of the bath, causing a separation of bismuth in the bath, and producing a dry bismuth-containing dross on the bath by incorporating in the bath a material selected from the group consisting of proteins, carbohydrates, and organic acids yielding carbon on thermal decomposition thereof.

8. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, maintaining the bath at substantially as low a temperature as is consistent with fluidity of the bath, causing a separation of bismuth in the bath, and producing dry bismuth-containing dross on the bath by incorporating in the bath small but efiective amounts of a protein substance.

9. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, maintaining the bath at substantially as low a temperature as is consistent with fluidity of the bath, causing a separation of bismuth in the bath and producing a dry bismuth-containing dross on the bath by incorporating in the bath small but effective amounts of a carbohydrate.

10. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, maintaining the bath at substantially as low a temperature as is consistent with fluidity of the bath, causing a separation of bismuth in the bath, and producing a dry bismuth-containing dross on the bath by incorporating in the bath a small but eiTective amount of an acid thermally decomposable at the temperature of the bath to yield carbon in the bath.

11. The "process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, maintaining the bath at substantially as low a temperature as, is consistent with fluidity of the bath, causing a separation of bismuth in the bath, and producing a dry bismuth-containing dross on the bath by acting on the bath with active carbon.

12. The process of removing bismuth from.

bismuth-containing lead which comprises producing a molten bath of the lead, maintaining the bath at substantially as low a temperature as is consistent with fluidity, and producing a dry bismuth-containing dross on the bath by acting on the bath with carbon produced in situ in the bath.

13. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath of the lead, eiiecting a preliminary removal of bismuth from the lead at least to a condition of a working equilibrium, and efiecting further separation of bismuth from the molten lead by maintaining the bath at a temperature range of from approximately 630 F. to approximately 640 F., and converting residual bismuth in the bath into a readily removable condition by adding to the bath a modii'ying agent for changing the physical characteristics of the residual bismuth from a diilicult- 1y separable condition to a readily removable grzhdross, and removing the dry dross from the M. Theprocess of removing bismuth from bismuth containing lead which comprises producing a molten bath of the lead, effecting at least a preliminary removal of bismuth from the lead, and causing further separation of bisuth from the lead by maintaining the bath at a temperature range of from approximately 630 F. to apprordmately 640 F., and converting residual bismuth in the bath into a readily removable condition by adding thereto an organic material thermally decomposable at the temperature of the bath to yield carbon in the bath thereby converting the bismuth in the bath into a dry dross, and removing the dross from the bath.

15. The process of removing bismuth from bismuth-containing lead which comprises producing a molten bath oi the lead, eflecting at least a preliminary removal of bismuth from the lead, and causing further separation of bismuth from the lead by maintaining the bath at a temperature range 01' from approximately 630 F. to approximately 640 F., and converting residual bismuth in the bath into a readily removable condition by adding to the bath a material selected from the class of compounds consisting of proteins, carbohydrates and acids thermally decomposable at the temperature of the bath with production of free carbon.

16. The process for debismuthizing lead which comprises imparting an alkali, alkaline earth metal content to the lead, controlling the temperature of the lead within a range slightly above its melting point, incorporating in the lead a reagent yielding free carbon by thermal decomposition in situ at such temperature, and separating the resulting dross and the lead.

17. The process for treating lead containing bismuth which comprises initially debismuthizing the lead with reagent from the class consisting of alkali, alkaline earth metals, and further debismuthizing the lead so refined by incorporating therein, at a temperature approximating the melting point of the lead, organic reagent thermally decomposable at such temperature to yield free carbon within the body of the lead thereby to form a dry dross which contains bismuth and is readily separable from the final refined lead.

18. The process for treating lead containing bismuth which comprises initially debismuthizing the lead with reagent from the class consisting of alkali, alkaline earth metals, and further debismuthizlng the leadso refined by incorporating therein, at temperatures approximating the melting point of the lead, successive additions of organic reagent thermally decomposable at such temperatures to yield free carbon within the body of the lead thereby to form a dry dross which contains bismuth and is readily separable from the l refined lead.

19. The process for further refining lead which has been treated with reagents from the class consisting of alkali, alkaline earth metals for the removal of bismuth and which still contains such reagents comprising: establishing a molten bath of the lead at a temperature only slightly above its melting point, incorporating in the bath a reagent thermally decomposlng therein to release tree carbon in situ, and eiiecting a separation between the resulting dross and the main body of lead. 20. The process for refining lead with respect to bismuth as a contaminant which comprises forming the bismuth as a solid phase dispersed throughout the body of a molten bath of the lead by incorporating therein reagent embraced by the alkali, alkaline earth groups, maintaining the molten bath at a temperature not materially higher than 640 R, and incorporating an organic reagent in the bath yielding free carbon by decomposition therein at said temperature thereby segregating the aforesaid bismuth in solid phase as a dry dross amenable to separation from the bath.

JESSE 0. BE'I'IERTON. YURII E. LEBEDEFF. 

